Dielectric resonators are formed of bodies of low loss, temperature stable, high permittivity, slab-like ceramic bodies. They are resonately responsive to specific electrical frequencies of current carried by adjacent circuit conductive elements. Their resonant frequency is dependent upon their shape, size, proximity to adjacent circuit elements and the dielectric constant of the body forming the resonator which exceeds that of the surrounding environment.
Insomuch as it is impractical to provide a multitude of dielectric resonators of diverse sizes and shapes for selective incorporation into circuits, dielectric resonators have been provided with means for tuning their frequency of resonant response after they have been built into circuits. Heretofore, such resonators have been tuned or trimmed by providing a positionally adjustable, low impedence element in close proximity with the body of dielectric material. For example, in U.S. Pat. No. 2,890,422 a metallic trimmer disc is positioned closely adjacent a dielectric resonator and mechanically reoriented with respect to the resonator for tuning its resonant frequency. As disclosed by this same patent, tuning may also be accomplished by imposing a magnetic bias on the dielectric resonator which bias may be adjusted by altering the electric current passing through a coupling coil.
Coils for generating magnetic fields of varying flux density have also been used for tuning resonant frequencies of ferrite type resonators, as disclosed in U.S. Pat. No. 3,766,494, and for tuning YIG oscillators and filters as shown in U.S. Pat. No. 4,096,461. Nevertheless, today dielectric resonators are probably still most commonly tuned by mechanical relocation of adjacent elements as exemplified by U.S. Pat. No. 4,484,162 which issued in Nov. 20, 1984.
With today's microminiaturization of electronic circuits, the use of mechanically adjustable means for tuning dielectric resonators has become increasingly impractical. The sizes of the various conductive elements of microstrip circuits, for example, are simply too small to include such. Accordingly, more than ever a need exists for dielectric resonators with tuning means that do not necessitate the use of mechanically relocatable or adjustable elements. Thus, it is to the provision of a tunable dielectric resonator that does not require the use of such movable elements that the present invention is primarily directed.